where should i mark a seam onto my mesh?
You need to put the seams on a part that won't be visible on rendering. You can't avoid discontinuity with this kind of object, unless you give corrections afterwards and recreate continuity with a Clone brush for example, but it will depend on the nature of your pattern, if it's a big pattern (like big flowers) it seems hard to achieve.
As you can see here I put the seam along the back of the cube, so that it will be hidden in render. For this kind of pattern it would be very hard to create a continuous texture: